1. Field of the Invention
This invention relates to a mobile radio communication system and a radio circuit controlling method therefor.
2. Description of the Related Art
Conventionally, in a mobile radio communication system which includes a plurality of base stations and a mobile station which can communicate with the plurality of base stations individually by radio, each of the base stations has a limited region (wireless zone) in which it provides services, and in principle, the mobile station communicates with one of the base stations which has a wireless zone in which the mobile station is currently present. However, in order to eliminate places in which the mobile station cannot communicate, the region in which each base station can communicate with the mobile station is actually set somewhat wider. Consequently, there actually is a region in which the mobile station can communicate with a plurality of base stations.
In such a conventional mobile ratio communication system as described above, communication is sometimes disabled when the traffic in a wireless zone of a base station is so great as to cause congestion of communication channels. Describing more particularly, in a mobile ratio communication system which employs a time division multiplex access (TDMA) mobile radio communication system, the congestion of communication channels is restricted by the number of radio circuits physically arranged in the wireless zone.
Therefore, conventionally, in order to establish a call connection between a certain base station and a mobile station which is present in the wireless zone of the base station, the mobile station measures the reception level of a radio wave transmitted from the base station to which a call connection is to be established and further measures the reception level of other radio wave transmitted from other base stations which are positioned around the wireless zone of the base station to which a call connection is to be established. The reception levels of the received signals measured by the mobile station are reported to the base stations.
Meanwhile, a host apparatus of the base stations supervises whether or not congestion of communication channels occurs in the wireless zone of each of the base stations, and if congestion of communication channels is detected and a free radio circuit is not available, the host apparatus confirms the reception levels reported from the mobile station which is communicating with the base station which has the wireless zone. If the host apparatus confirms, as a result of the confirmation of the reception levels, that the reception level of a radio wave transmitted from another base station is sufficiently high for the quality of the radio circuit to be secured, then it controls so that radio communication may be performed between the mobile station and the another base station although the another base station has a wireless zone different from the wireless zone set originally for the place.
Here, in a time division mobile radio communication system, for communication between a mobile station and a base station, the same frequency is used in a plurality of wireless zones at positions spaced away from each other by a distance larger than a distance set in advance. This presupposes that each base station communicates with mobile stations which are present in a wireless zone set for the base station, and is intended to achieve effective utilization of frequencies.
However, if, when congestion of communication channels occurs in a wireless zone, communication is performed between a mobile station and another base station ignoring the wireless zone division, then the communication is performed using the same frequency at positions spaced from each other by a smaller distance than the prescribed distance. Consequently, there is the possibility that interference may occur and cause failure of the communication.
On the other hand, in a mobile radio communication system which adopts a code division multiplex access (CDMA) mobile radio communication (CDMA) system, a radio wave transmitted from a mobile station becomes an interfering wave with another mobile station, resulting in deterioration of the communication quality of the latter mobile station.
Therefore, a base station measures the reception power in the reverse-link of a radio wave transmitted from a mobile station and then produces, based on a result of the measurement, a control signal for controlling the transmission power from the mobile station to a minimum value necessary to obtain a required quality, and transmits the control signal to the mobile station. As a result, the transmission power from the mobile station is controlled to the minimum value necessary to obtain the required quality.
Here, in a mobile radio communication system which adopts such a code division multiplex mobile radio communication system as described above, when a mobile station is present in an area in which the wireless zone of a certain base station and the wireless zone of another base station overlap with each other, the mobile station communicates with both of the two base stations and thus receives control signals transmitted from the two base stations. In this instance, the transmission power is set in accordance with that one of the received control signals which controls the transmission power to a lower value.
Normally, the transmission power is in most cases controlled to a lower value by a nearer base station.
By the way, if a mobile radio communication system which adopts the code division multiplex mobile radio communication system adopts a radio circuit controlling system adopted by the time division mobile radio communication system described above, then when congestion of communication channels occurs, a mobile station communicates not with the nearest base station but with another base station spaced by a larger distance, and consequently, a higher transmission power is required. In this instance, a radio wave transmitted from the radio station becomes an interfering wave with other mobile stations. Consequently, there is a problem in that the communication quality of the other mobile station cannot be maintained, resulting in a deterioration of service.
In this manner, the controlling method upon communication channel congestion adopted by such a time division mobile ratio communication system as described above conflicts with the controlling method in an ordinary code division multiplex mobile radio communication system.
By the way, as described above, a radio wave of a mobile station becomes an interfering wave with other mobile station principally in the reverse-link in which the radio wave is transmitted from the mobile station.
Further, actually the traffic of the forward-link and the traffic of the reverse-link in the wireless zone of a base station are sometimes different from each other (asymmetrical communication condition). For example, in data communication of the internet, the traffic in the forward-link is greater than the traffic in the reverse-link, and congestion of communication channels sometimes occurs only in the forward-link.
Here, in a controlling method adopted by such a time division mobile radio communication system as described above, if congestion of communication channels occurs, then since a mobile station is controlled so that it communicates with another base station in both the reverse-link and the forward-link, if congestion of communication channels occurs only in the forward-link, the mobile station is controlled so that also the communication thereof in the reverse-link in which no congestion of communication channels occurs is performed with the another base station, resulting in the possibility that the communication quality of the latter mobile station may be deteriorated.
Therefore, a mobile radio communication system which adopts the code division multiplex mobile radio communication system does not adopt a controlling method similar to that of the time division mobile radio communication system because interference occurs between mobile stations.
Further, where physical radio circuits are present, when the traffic exceeds a fixed level, control to prevent new communication from being started is performed.